1. Field of the Invention
This invention relates to the field of optical fiber connectors that permit introduction of a laser beam into an optical fiber.
2. Brief Description of Related Prior Art
A functional diagram of a device for injecting a laser beam into an optical fiber is shown in FIG. 1. As shown in this diagram, the device is made up of:
a convergent optical system 2 for allowing an incident laser beam 4 to be focused onto the input surface 6 of a fiber 8, PA1 an adjustable mechanical coupling 10 (an interface collar) between the optical system 2 and the fiber connector 12, that permits adjustments for centering the focused beam on the input surface of the fiber, PA1 and finally an optical fiber connector 12, which has a first fliction of creating a link between an optical component (the end at the optical fiber) and the mechanical coupling 10. PA1 the energy per pulse (for example from 1 to 140 J), PA1 the duration of the pulse (for example from 0.5 to 20 ms), PA1 the repetition frequency (for example from 1 to 500 Hz) PA1 the mean power (W) (energy per pulse).times.(repetition frequency), PA1 the energy per pulse (J), PA1 the peak power (kW) (energy per pulse)/(duration of the pulse). PA1 mean power: 900 W, PA1 energy per pulse : 90 J PA1 peak power: 18 kW. PA1 the laser energy parameters which slowly damage the input surface of the fiber, PA1 the possible deposition of dust on the input surface of the fiber, creating local hot spots, PA1 a slight axial defocusing of the beam on the input surface of the fiber, which results in heating of the periphery of the fiber, PA1 a slight transverse defocusing of the beam on the input surface of the fiber, which results in an asymmetrical temperature rise on one edge of the fiber, PA1 residual micro scratches, arising from previous polishing, which reduce the life of the input surface, PA1 residual traces of solvent, arising from previous cleaning, which reduce life of the input surface of the fiber. PA1 two half grips for receiving one end of the optical fiber in a gripped position, PA1 a connector body for receiving the two half grips and for holding them in an optical fiber gripped position PA1 cooling means for cooling the two half grips.
Document U.S. Pat. No. 5,291,570 describes a fiber connector for a continuous high energy laser shown diagrammatically in FIG. 2. A laser beam 14 passes into an optical fiber 16 through one of its ends. The external part of the connector is made up of a body 18. The end of the fiber is chamfered and protected by a sleeve 20. The sleeve is made of a material that is transparent to the wavelength of laser beam 14. Reference number 22 designates the area of contact of the sleeve with the end of the fiber.
Beyond this contact area 22, the fiber 16 and the sleeve 20 are separated by an air gap 24. The fraction of the beam 14 that does not pass into the interior of the fiber passes through the transparent sleeve 20. This parasitic beam is reflected by component 26 towards the body 18. This body is built of a material that is a good conductor of heat.
Document DE-40 28305 describes a fiber connector for a high energy laser. This device has the objective of automatically centering the end of the fiber with respect to the incident beam. A special bimetallic grip, with three branches, exerts a radial force on the end of the fiber when one of the branches receives a greater fraction of the beam. Hence the centering of the beam is automatic.
Although satisfactory in certain respects, these two devices nevertheless pose problems, particularly when high energy lasers are used, for example, a pulsed YAG laser. The YAG laser emits a beam with a wavelength of 1.06 .mu.m, which wavelength can be transported by an optical fiber. The diameter of the fibers used is, in general, 1 mm for a YAG 1 kW pulsed laser.
Compared with a continuous YAG laser, the mean power of the industrial YAG pulsed laser is actually less (1 to 2 kW). Even so, the quality of its beam is not so good.
The YAG pulsed laser is characterised by three parameters which control the laser pulse:
The different "energy values" characterizing the pulsed YAG laser beam are:
The technology of the YAG pulsed laser is particularly interesting since it is possible to modulate and optimize the pulse parameters for each application case and to attain good performance for scribing, welding, surface treatment and drilling.
For example, if the beam is transmitted via optical fiber (of 1 mm diameter), it is possible to cut a 30 mm stainless steel plate, when assisted by an oxygen jet, if the beam has the following parameters:
Energy parameters even higher than those shown above are envisaged in the field of dismantling nuclear installations by YAG pulsed laser.
For example, with direct firings (that is to say, without transport of the beam by an optical fiber), at 1300 W mean power, 130 J per pulse and 22 kW of peak power, it is possible to achieve a cut thickness of 50 mm (304 L stainless steel), with good cutting quality.
On the other hand, continuous progress in the technology of the industrial YAG laser has allowed the achievement today of mean powers of 2000 W and energies per pulse of 200 J. Three thousand W prototypes are in the process of being developed.
If a pulsed YAG laser beam, having increased energy parameters, is used in combination with an optical fiber, the end of which is held in a connector of the type described in the prior art, then the life of the fiber input surface is very short. Hence for a mean power of 900 W, an energy per pulse of 90 J and a peak power of 18 kW, the life of the input face of the fiber is about 5 minutes. Heating of the fiber to its softening temperature is also observed, and the input surface of the fiber sublimes little by little until its complete destruction occurs. It is therefore necessary to provide an optical fiber connector that allows resolution of this problem.
Furthermore, a maintenance problem is also posed for the input surface of the fiber which, even if it is not destroyed needs to be checked, and even repolished. The reasons why repeat polishing of the input surface is necessary are manifold. They are, for example:
These different phenomena necessitating repolishing the input surface of the fiber are, in practice, more or less cumulative and difficult to identify, especially when the input surface of the fiber is really damaged.
Consequently, the fiber must be capable of being dismantled from its connector before being repolished, and then reassembled with its connector after polishing. During this operation, it is necessary to avoid as much as possible, any risk of damaging the input surface of the fiber, e.g. chipping of the fiber that can arise if the input surface comes into contact with another element of the optical device.